This invention relates to an apparatus for the noninvasive measurement of liquids within a reservoir. In particular, the invention relates to a mechanism for measuring the height of a liquid and/or the occurrence of a liquid draining below a certain point so as to warn the user when additional liquid must be added or the reservoir replaced.
Medical science often requires that liquids be administered to a patient in a variety of situations. These liquids include simple intravenous feeding solutions, saline solutions for providing pressure to the eye during ocular surgery, contrast media infused to enhance imaging abilities, and blood administered during transfusions. In virtually all such situations, it could be dangerous for the liquid supply to "inadvertently run dry." In some applications, allowing the reservoir to run dry may decrease the pressure on the liquid below that desired. In other situations, it can result in air entering the blood stream, causing complications or even death.
Several approaches have been suggested for monitoring reservoirs of liquids so that inadvertent "running dry" can be avoided. For example, a system marketed by Trek Medical in Mukwanago, Wis., involves use of an electrical needle skewered into the bottom of a bottle containing liquid to be monitored. A constant electrical current is applied to the needle, and when the liquid level drops below the end of the needle, the break in the electrical current causes a lamp to light on a master control panel.
While the Trek Medical system works, it has several disadvantages. First, a separate needle is generally used for each monitored bottle. Second, the fixed location of the needle results in the fixed triggering position. Thus, the user cannot select the liquid level at which the system will signal that the reservoir needs to be refilled or replaced. While a fixed location may be desirable for some uses, the desired triggering position of the indicator may vary for different medical procedures. Additionally, some procedures may benefit from being able to vary the location at which the indicator signal reacts during different periods of the procedure. A third disadvantage of the above-mentioned system is that the placement of the needle risks the introduction of microbes and other contaminants that are generally undesirable in medical procedures.
Other available systems, marketed by Sarns, Inc. and CGH Medical, have ultrasonic liquid level detectors for blood reservoirs in which the transducer is placed against an exterior sidewall of the reservoir. Ultrasonic signals are emitted into the reservoir and reflected signals are used to determine when the liquid level has dropped below a designated point. The coupling between the transducer and reservoir sidewall, however, requires that gel be placed on the sidewall to conduct the ultrasonic signals from the transducer into the reservoir and from the reservoir back into the transducer. This approach can be time consuming and messy as gel must be applied to the transducer or sidewall each time the two are coupled.